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Michigan CDL Doubles/Triples study materials
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Michigan CDL Doubles/Triples practice questions

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Question 1Coupling and Uncoupling

When coupling a tractor to the first semitrailer of a doubles set, at what height should the trailer's upper coupler plate sit relative to the fifth wheel before you back under it?

  1. Slightly below the fifth wheel so the tractor lifts it during coupling Correct answer.
  2. As low as possible so the landing gear stays on the ground
  3. High enough that the trailer plate rides just above the fifth wheel
  4. Level with the fifth wheel so the jaws slide straight in

Explanation

The upper coupler plate should sit slightly below fifth-wheel height so the tractor lifts the trailer slightly as it backs under. This helps the plate seat on the fifth wheel and the kingpin engage correctly. A trailer that is too low or too high can fail to couple properly.

Question 2Coupling and Uncoupling

After backing under a trailer and hearing the fifth wheel jaws close, what must you do before releasing the tractor parking brake to pull a test tug?

  1. Raise the trailer axle suspension to maximum height
  2. Chock the trailer wheels and disconnect the glad hands
  3. Set the trailer spring brakes by pulling the trailer air supply valve out Correct answer.
  4. Lower the landing gear fully to the ground

Explanation

You must supply air to the trailer and set its spring brakes so the trailer cannot roll when you tug against it. Pulling the trailer air supply valve out charges the trailer system and applies the trailer brakes. Only then does a gentle pull in low gear prove the jaws are locked.

Question 3Coupling and Uncoupling

You have completed a successful test tug on a converter dolly and trailer combination. What is the correct next step for the landing gear?

  1. Leave it down until you have driven one mile
  2. Crank it fully up and secure the crank handle Correct answer.
  3. Lower it slightly to stabilize the trailer
  4. Remove the crank handle and store it in the cab

Explanation

Once the tug confirms the coupling, crank the landing gear fully up so it cannot drag or catch on railroad tracks and pavement. Secure the crank handle so it does not swing loose in transit. Leaving gear down will damage it and can cause loss of control.

Question 4Coupling and Uncoupling

When uncoupling a set of doubles, why should you lower the landing gear to the ground before releasing the fifth wheel jaws?

  1. To keep the trailer level while you disconnect the electrical cord
  2. To raise the converter dolly off its tires
  3. To apply the trailer spring brakes automatically
  4. So the trailer is supported and does not drop when the tractor pulls out Correct answer.

Explanation

The landing gear must carry the trailer's weight before the fifth wheel is released, otherwise the front of the trailer drops when the tractor pulls away. Crank the gear down until it firmly contacts the ground. Only then should you pull the release handle.

Question 5Coupling and Uncoupling

In a doubles or triples combination, what component connects the converter dolly to the semitrailer ahead of it?

  1. The safety chains running under the dolly
  2. The pintle hook on the rear of the lead trailer
  3. The fifth wheel mounted on the converter dolly Correct answer.
  4. The glad hands between the trailers

Explanation

The converter dolly carries its own fifth wheel, which accepts the kingpin of the trailer behind it. The dolly's drawbar attaches to the pintle hook of the trailer ahead. Understanding this split matters because coupling checks apply at both the pintle hook and the dolly fifth wheel.

Question 6Coupling and Uncoupling

What does the locking lever or safety latch on a pintle hook do for a doubles coupling?

  1. It aligns the dolly fifth wheel with the kingpin
  2. It sets the trailer spring brakes
  3. It keeps the dolly drawbar ring from bouncing off the hook Correct answer.
  4. It adjusts the air pressure to the rear trailer

Explanation

The pintle hook latch closes over the drawbar ring so the ring cannot jump off the hook during travel. Always confirm the latch is fully closed and secured after coupling. A missing or open latch allows the dolly to separate from the lead trailer.

Question 7Coupling and Uncoupling

Before backing a tractor under a converter dolly's fifth wheel, what should you verify about the dolly?

  1. That its wheels are chocked and its spring brakes are released
  2. That it is level, its wheels are chocked, and its spring brakes are set Correct answer.
  3. That its drawbar is disconnected from the lead trailer
  4. That its landing gear is fully raised

Explanation

A dolly must be held still and level so the tractor can engage its fifth wheel without the dolly rolling or tipping. Chock the wheels and confirm the spring brakes are applied. Then back under slowly and check the jaws.

Question 8Coupling and Uncoupling

Where should you stand when checking that the fifth wheel jaws have closed around a kingpin?

  1. Under the trailer deck near the kingpin
  2. Directly behind the trailer between the tandems
  3. In the cab watching the dash indicator
  4. Beside the tractor frame rail looking at the jaw opening Correct answer.

Explanation

Get beside the tractor and look up into the fifth wheel throat to see the jaws wrapped around the kingpin. Never put yourself under or between the units where a rollaway could crush you. Visual confirmation plus a test tug is the accepted check.

Question 9Coupling and Uncoupling

You are coupling a tractor to a lead trailer that will pull two pups. After the test tug succeeds, you notice the trailer is sitting noticeably nose-high. What is the concern?

  1. The fifth wheel height is mismatched, which can overload the rear axle and lighten the steer axle Correct answer.
  2. The kingpin is too short for the jaws
  3. The trailer spring brakes will not release at that angle
  4. The electrical cord will chafe on the deck

Explanation

A nose-high trailer means the fifth wheel plate height does not match the trailer, shifting weight rearward and reducing steering axle load. This hurts steering traction and can overload trailer or drive axles. The combination should be checked on a scale and the height corrected before dispatch.

Question 10Coupling and Uncoupling

During a test tug on a newly coupled trailer, the tractor moves forward but the trailer stays put and you hear a clunk. What is the most likely problem?

  1. The landing gear is still touching the ground
  2. The glad hands are crossed
  3. The trailer spring brakes are released and the wheels are rolling
  4. The fifth wheel jaws did not lock around the kingpin Correct answer.

Explanation

If the tractor pulls out from under the trailer with a clunk, the jaws never captured the kingpin. The trailer staying in place while the tractor moves is the classic sign of a high or missed couple. Reset, lower the trailer plate to just above the fifth wheel, and recouple.

Question 11Coupling and Uncoupling

When uncoupling the rear trailer of a set, in what order should you release the connections at the converter dolly?

  1. Pull the fifth wheel release handle, then lower the landing gear
  2. Disconnect the air lines first, then lower the landing gear
  3. Lower the landing gear, then pull the fifth wheel release handle Correct answer.
  4. Release the pintle hook, then pull the fifth wheel handle

Explanation

Support the trailer on its landing gear before releasing the fifth wheel so it cannot drop onto the dolly or ground. After the gear is set, pull the release handle and pull the tractor or dolly away slowly. Air lines and electrical cord are disconnected after the trailer is supported.

Question 12Coupling and Uncoupling

You are coupling a converter dolly to the pintle hook of a lead trailer. Which check confirms the connection is secure?

  1. The dolly fifth wheel is greased
  2. The dolly's air lines reach the trailer without stretching
  3. The dolly wheels spin freely by hand
  4. The dolly drawbar ring sits fully in the hook with the latch closed Correct answer.

Explanation

The drawbar ring must seat fully down in the pintle hook with the safety latch closed and secured. Give a light pull to confirm the ring will not lift out. Air line reach, wheel spin, and grease are unrelated to mechanical security.

Question 13Coupling and Uncoupling

After coupling a dolly and trailer, you find the trailer's front air line and electrical cord will not reach the dolly receptacles. What is the proper action?

  1. Run the lines under the trailer frame to gain slack
  2. Loop the lines over the dolly drawbar to shorten them
  3. Use the correct-length coiled air lines and cord rated for the application Correct answer.
  4. Stretch the lines tight and secure them with tape

Explanation

Air lines and cords must be the proper length for the combination and long enough to allow full articulation without tension. Stretching them tight causes chafing and separation, which can trigger a trailer brake lockup. Never route lines where they can be pinched or dragged.

Question 14Coupling and Uncoupling

When backing a tractor under a trailer, why should you approach the kingpin slowly and stop when the fifth wheel contacts the trailer plate?

  1. To let the air build in the trailer tanks
  2. To allow the landing gear to retract automatically
  3. To avoid striking the kingpin and bending the fifth wheel or trailer frame Correct answer.
  4. To give the jaws time to cool

Explanation

Backing too fast can ram the kingpin and damage the fifth wheel, kingpin, or trailer structure. Approach slowly and let the fifth wheel lift the trailer as the jaws ride over the pin. A controlled stop at contact lets you verify alignment before the final push.

Question 15Coupling and Uncoupling

You must uncouple a loaded set on a slight slope. What precaution is most important before releasing any fifth wheel?

  1. Set the tractor and trailer parking brakes and chock the trailer wheels Correct answer.
  2. Drain all air from the trailer tanks
  3. Remove the dolly drawbar from the pintle hook first
  4. Turn the steering wheels toward the curb and leave the engine running

Explanation

On any grade, set the parking brakes on the tractor and trailers and chock the wheels of the unit you are leaving. This prevents a rollaway once the mechanical connection is released. Never rely on air pressure alone, since it can bleed off.

Question 16Coupling and Uncoupling

What is the purpose of the trailer air supply valve (the red button) during coupling and uncoupling?

  1. It balances air pressure between the two trailers only
  2. It adjusts the fifth wheel jaw tension
  3. It controls the tractor's engine brake
  4. It charges or shuts off air to the trailer, setting or releasing its spring brakes Correct answer.

Explanation

Pushing the red trailer air supply valve in charges the trailer system and releases its spring brakes; pulling it out shuts off supply and applies them. During coupling you push it in to charge the trailer and set brakes for the tug. During uncoupling you pull it out to secure the trailer before disconnecting lines.

Question 17Coupling and Uncoupling

After uncoupling a trailer and pulling the tractor away, you notice the trailer's front air lines are still connected to the tractor and have been stretched. What should you have done differently?

  1. Disconnected the glad hands and coiled the lines before pulling away Correct answer.
  2. Released the fifth wheel before lowering the landing gear
  3. Backed the tractor straight out without turning
  4. Left the tractor air supply valve pushed in during uncoupling

Explanation

Air lines and the electrical cord must be disconnected and stowed before the tractor moves away from a dropped trailer. Pulling away with lines attached stretches or tears them and can damage the couplers. Build the disconnect into the uncoupling sequence right after the trailer is supported and jaws released.

Question 18Coupling and Uncoupling

Which condition indicates the trailer kingpin may be worn or damaged after coupling?

  1. The glad hands couple without leaking
  2. The landing gear cranks easily
  3. The jaws close but the trailer rocks fore-and-aft noticeably on the fifth wheel Correct answer.
  4. The trailer sits level with the tractor

Explanation

Excessive fore-and-aft rock after a confirmed lock suggests a worn kingpin, worn jaws, or a fifth wheel that is not fully seated. A properly locked kingpin holds the trailer firmly with minimal play. Have the components inspected before operating.

Question 19Coupling and Uncoupling

When coupling the second trailer of a doubles set, which connection carries the trailer's kingpin load?

  1. The safety chains between the units
  2. The fifth wheel on the converter dolly Correct answer.
  3. The pintle hook on the first trailer
  4. The electrical cord

Explanation

The second trailer's kingpin locks into the fifth wheel mounted on the converter dolly, which carries the vertical load. The dolly drawbar and pintle hook connect the dolly to the first trailer. Each connection must be verified separately.

Question 20Coupling and Uncoupling

Before coupling a set, you inspect the fifth wheel and find the jaws will not open fully. What is the correct action?

  1. Correct the jaw or release mechanism before coupling Correct answer.
  2. Couple and rely on the test tug to seat the jaws
  3. Force the jaws open with a pry bar and proceed
  4. Apply grease and back under the trailer anyway

Explanation

A fifth wheel that will not open fully cannot be trusted to lock correctly around the kingpin. Repair or adjust the release mechanism before attempting to couple. Forcing or ignoring the problem risks a false lock and trailer separation.

Question 21Coupling and Uncoupling

During uncoupling, why should you pull the tractor forward only a short distance after releasing the fifth wheel, then stop and check?

  1. To keep the landing gear from sinking into soft ground
  2. To let the trailer air tanks recharge
  3. To allow the jaws to reset for the next coupling
  4. To confirm the trailer stays supported and nothing is still attached Correct answer.

Explanation

Pulling forward slowly lets you confirm the trailer remains stable on its landing gear and that no lines, chains, or pins are still connected. If something is still attached, you can stop before it is damaged. This controlled check is safer than driving off immediately.

Question 22Coupling and Uncoupling

A driver couples a set and the test tug succeeds, but the rear trailer drags its landing gear on dips. What is the most likely cause?

  1. The dolly fifth wheel is over-greased
  2. The landing gear was not cranked fully up after coupling Correct answer.
  3. The trailer spring brakes are dragging
  4. The pintle hook latch is open

Explanation

Landing gear left partially down will contact the road on dips and can be torn off. After a successful tug the gear must be cranked fully up and the handle secured. Dragging gear is a coupling-completion failure, not a brake or latch problem.

Question 23Coupling and Uncoupling

You are coupling on uneven ground and the trailer plate sits lower on one side. What is the safest approach?

  1. Reposition the tractor or trailer so the units align before coupling Correct answer.
  2. Back under quickly so the jaws snap over the kingpin
  3. Couple anyway and correct the lean with the suspension leveling valve
  4. Use the landing gear crank to tilt the trailer toward the tractor

Explanation

Misalignment from uneven ground can make the jaws miss the kingpin or damage the fifth wheel. Reposition so the tractor and trailer are aligned and the plate sits just above the fifth wheel. Never force a couple on a mismatch and expect the suspension to fix it.

Question 24Coupling and Uncoupling

After coupling a triple set, you find the rear trailer's spring brakes will not release even though the red supply valve is in. What is the most useful first check at the coupling points?

  1. Whether the glad hands are fully seated and the trailer supply line is connected Correct answer.
  2. Whether the dolly fifth wheel is greased
  3. Whether the landing gear crank is stowed
  4. Whether the pintle hook latch is closed

Explanation

If the trailer will not release its spring brakes, the supply line is likely not delivering air, often because a glad hand is not fully seated or a coupler is damaged. Check each glad hand connection along the set. Latch, grease, and crank stowage do not affect air delivery.

Question 25Coupling and Uncoupling

Why is a visual check of the jaws plus a test tug considered more reliable than either check alone when coupling doubles?

  1. The tug releases the spring brakes so the visual check becomes unnecessary
  2. The visual check is required by law and the tug is optional
  3. The tug alone cannot move the tractor in low gear
  4. The visual check confirms the jaws wrapped the kingpin, and the tug proves the lock holds under load Correct answer.

Explanation

Looking into the throat confirms the jaws are actually around the kingpin rather than closed on air, while the tug proves the lock holds under pulling force. Each check catches a different failure mode, so together they give much stronger assurance. Relying on one alone has allowed high couples to go undetected.

Question 26Coupling and Uncoupling

When coupling a tractor to a converter dolly, what must be true of the dolly's landing gear before you back under it?

  1. It must be removed from the dolly entirely
  2. It must be lowered so the dolly rests on the ground
  3. It must be cranked down until the wheels clear the ground
  4. It must be fully raised and locked in the up position Correct answer.

Explanation

A converter dolly's landing gear (support legs) must be fully raised and secured before coupling so the legs cannot catch on the ground or interfere with the fifth wheel as the tractor backs under. If the legs are down, the dolly cannot settle properly onto the tractor's fifth wheel.

Question 27Coupling and Uncoupling

What is the primary purpose of the safety latch (locking lever) on a pintle hook used to tow a converter dolly?

  1. To adjust how much the dolly can swivel
  2. To keep the drawbar eye from bouncing off the hook Correct answer.
  3. To hold the dolly's air lines in place
  4. To lock the dolly's wheels during coupling

Explanation

The safety latch closes over the pintle hook opening so the drawbar eye cannot bounce or lift off the hook while towing. Without it latched, the eye can separate from the hook on bumps, and the dolly can break away.

Question 28Coupling and Uncoupling

When you back the tractor under a trailer that will pull doubles, at what point should the fifth wheel contact the trailer apron?

  1. When the trailer's landing gear is fully raised
  2. When the trailer's rear wheels touch the ground
  3. When the fifth wheel is just under the nose of the trailer Correct answer.
  4. When the kingpin is fully inside the jaws

Explanation

You stop when the fifth wheel is just under the trailer nose so the kingpin can enter the throat of the fifth wheel as you continue backing. Backing too far before stopping can strike the trailer and damage the kingpin or the tractor.

Question 29Coupling and Uncoupling

Which connection on a converter dolly actually carries the weight and pivot load of the trailer behind it?

  1. The dolly's fifth wheel and kingpin Correct answer.
  2. The dolly's electrical cord
  3. The dolly's air supply line
  4. The pintle hook and drawbar eye

Explanation

The trailer behind the dolly rests on the dolly's fifth wheel, and its kingpin locks into the dolly's jaws. That fifth wheel/kingpin interface carries the vertical load and provides the pivot point for the trailing trailer.

Question 30Coupling and Uncoupling

Before coupling a set, you find the fifth wheel jaws will not open fully. What is the correct action?

  1. Couple the trailer and rely on the test tug to seat the jaws
  2. Leave the jaws closed and back under the trailer
  3. Force the jaws open with a pry bar and couple anyway
  4. Clean, lubricate, and repair the jaw mechanism before coupling Correct answer.

Explanation

Jaws that will not open fully cannot reliably close and lock around the kingpin, so the fifth wheel must be cleaned, lubricated, and repaired before coupling. Forcing or ignoring the problem risks an incomplete lock and trailer separation.

Question 31Coupling and Uncoupling

Why should you pull the tractor forward only a short distance after releasing the fifth wheel during uncoupling?

  1. So the trailer's brakes release automatically
  2. So the dolly's pintle hook opens
  3. So the trailer's air lines disconnect on their own
  4. So the trailer's landing gear is not dragged or bent Correct answer.

Explanation

Once the fifth wheel is released, the trailer is resting on its landing gear. Pulling forward only a short distance clears the tractor without dragging the trailer or bending the landing gear legs.

Question 32Coupling and Uncoupling

What is a key reason to chock the trailer wheels before uncoupling a set on any slope?

  1. To keep the trailer from rolling once the brakes are released Correct answer.
  2. To keep the air lines from kinking
  3. To make the landing gear easier to crank
  4. To help the fifth wheel jaws open

Explanation

When you release the trailer's spring brakes during uncoupling, nothing else holds the trailer in place on a slope. Chocking the wheels prevents it from rolling away and crushing or trapping you.

Question 33Coupling and Uncoupling

When coupling a set, what does a proper test tug confirm?

  1. That the fifth wheel jaws have locked around the kingpin Correct answer.
  2. That the landing gear is fully raised
  3. That the dolly's air lines are connected
  4. That the trailer's lights are working

Explanation

A test tug with the trailer brakes set pulls against the fifth wheel; if the jaws are locked on the kingpin, the tractor will not pull free. If the connection is incomplete, the tractor moves and the trailer stays put.

Question 34Coupling and Uncoupling

You are coupling a lead trailer to a converter dolly. After the test tug succeeds, what should you do next before driving away?

  1. Lower the dolly landing gear to the ground
  2. Raise the dolly landing gear and connect the air lines and electrical cord Correct answer.
  3. Release the tractor's spring brakes and drive off
  4. Remove the pintle hook safety latch

Explanation

After the fifth wheel connection is verified, raise the dolly's landing gear so it clears the road and connect the air lines and electrical cord so the trailing trailer's brakes and lights work. Driving off before these steps leaves the dolly unsupported or the trailer without brakes.

Question 35Coupling and Uncoupling

You are uncoupling the rear trailer of a doubles set. In what order should you release the connections at the converter dolly?

  1. Lower the landing gear, release the fifth wheel, then set the brakes
  2. Release the fifth wheel, then lower the landing gear, then disconnect the air lines
  3. Set the trailer brakes, lower the landing gear, disconnect the air lines and cord, then release the fifth wheel Correct answer.
  4. Disconnect the air lines, release the fifth wheel, then set the brakes

Explanation

You first set the trailer brakes and lower the landing gear so the trailer is supported, then disconnect the air lines and electrical cord by hand, and finally release the fifth wheel. Releasing the fifth wheel before the trailer is supported lets it drop.

Question 36Coupling and Uncoupling

After coupling a dolly to a trailer, the trailer's front air line and electrical cord will not reach the dolly receptacles. What is the safest action?

  1. Connect only the electrical cord and skip the air line
  2. Leave them disconnected and drive slowly
  3. Use the correct-length lines and cords rated for the application Correct answer.
  4. Stretch the lines as far as they will go and tie them off

Explanation

Air lines and cords must be the correct length for the application so they reach without strain and do not drag or chafe. Stretching or leaving them disconnected creates brake and light failures and can cause the lines to pull apart in service.

Question 37Coupling and Uncoupling

You are backing a tractor under a trailer on a slight grade. Why should you approach slowly and stop when the fifth wheel contacts the apron?

  1. To let the landing gear raise itself
  2. To keep the trailer's brakes from releasing
  3. To avoid striking the trailer and damaging the kingpin or fifth wheel Correct answer.
  4. To allow the air lines to connect automatically

Explanation

Approaching slowly and stopping at contact prevents a hard strike that can bend the kingpin, crack the apron, or damage the fifth wheel. It also lets you confirm alignment before continuing to seat the kingpin.

Question 38Coupling and Uncoupling

After coupling a set, you find the rear trailer's spring brakes will not release even though the red supply valve is open. What is the most likely cause?

  1. The rear trailer's air lines are not fully connected or are crossed Correct answer.
  2. The fifth wheel jaws are not locked
  3. The dolly landing gear is still down
  4. The pintle hook latch is open

Explanation

Spring brakes release only when air reaches the trailer through properly connected lines. If the rear trailer's lines are loose, crossed, or not connected, the brakes stay applied even with the supply valve open.

Question 39Coupling and Uncoupling

You couple a set and the test tug succeeds, but the rear trailer drags its landing gear on dips. What is the most likely cause?

  1. The pintle hook safety latch is closed
  2. The rear trailer's landing gear was not fully raised Correct answer.
  3. The fifth wheel jaws are too tight
  4. The air lines are too short

Explanation

Landing gear that was not fully raised hangs low and strikes the road on dips. The test tug only verifies the fifth wheel lock, so the gear position must be checked separately before driving.

Question 40Coupling and Uncoupling

What does the trailer air supply valve (red button) do during coupling and uncoupling?

  1. It locks the fifth wheel jaws
  2. It controls air to the trailer, releasing or applying the trailer's spring brakes Correct answer.
  3. It opens the pintle hook
  4. It raises the landing gear

Explanation

The red trailer air supply valve controls air flow to the trailer's brake system. Pushing it in charges the trailer and releases the spring brakes; pulling it out exhausts the air and applies the spring brakes for parking.

Question 41Coupling and Uncoupling

After uncoupling a trailer and pulling the tractor away, you notice the trailer's front air lines are still connected to the tractor. What should you do?

  1. Continue driving and let the lines drag
  2. Cut the lines free and drive on
  3. Back up and reconnect the lines to the dolly
  4. Stop, reconnect the lines to the trailer holders, and inspect for damage Correct answer.

Explanation

Air lines left connected will be stretched and torn as the tractor pulls away, causing brake failure and damage. You should stop, secure the lines on the trailer, and inspect them for damage before continuing.

Question 42Coupling and Uncoupling

Which condition after coupling suggests the trailer kingpin may be worn or damaged?

  1. The air lines reach the receptacles easily
  2. The trailer moves back and forth noticeably on the fifth wheel Correct answer.
  3. The pintle hook latch closes firmly
  4. The landing gear cranks smoothly

Explanation

Excessive movement of the trailer on the fifth wheel after coupling can indicate a worn or damaged kingpin or jaws. That play can allow the trailer to separate, so it must be inspected and repaired before driving.

Question 43Coupling and Uncoupling

You are coupling on uneven ground and the trailer apron sits lower on one side. What is the safest approach?

  1. Move to level ground or adjust the trailer so the fifth wheel aligns squarely Correct answer.
  2. Couple anyway and rely on the test tug
  3. Lower the landing gear on the high side to level the trailer
  4. Back under quickly to force the fifth wheel into place

Explanation

Coupling on uneven ground can misalign the fifth wheel and kingpin, causing an incomplete lock or damage. The safe approach is to move to level ground or adjust the trailer so the fifth wheel meets the apron squarely.

Question 44Coupling and Uncoupling

During a test tug on a newly coupled trailer, the tractor moves forward but the trailer stays put and you hear a clunk. What does this indicate?

  1. The pintle hook is properly latched
  2. The trailer brakes are working correctly
  3. The landing gear is fully raised
  4. The fifth wheel jaws did not lock around the kingpin Correct answer.

Explanation

If the tractor moves while the trailer stays put, the fifth wheel is not holding the kingpin, which means the jaws did not lock. You must stop, inspect the fifth wheel, and recouple before driving.

Question 45Coupling and Uncoupling

After coupling a triple set, you find the rear trailer's spring brakes will not release even though the red supply valve is open. Which cause should you check first?

  1. Fifth wheel jaws that are too tight
  2. A loose pintle hook latch
  3. A closed shut-off valve or disconnected line on the rear trailer Correct answer.
  4. Landing gear that is too high

Explanation

With the supply valve open, air must travel through every line and shut-off valve to the rear trailer. A closed shut-off valve or disconnected line blocks that air, so the spring brakes stay applied.

Question 46Coupling and Uncoupling

Why should you stand clear of the area between the tractor and trailer while the fifth wheel is being coupled?

  1. The air lines can whip around
  2. The landing gear can spring up
  3. The tractor can roll or the trailer can shift and crush you Correct answer.
  4. The pintle hook can open by itself

Explanation

A tractor can roll unexpectedly or a trailer can shift while the fifth wheel is being coupled, and anyone standing between them can be crushed. Staying clear of the gap removes that hazard.

Question 47Coupling and Uncoupling

You are uncoupling a set and the trailer's front air lines are still pressurized. What is the correct procedure?

  1. Leave the lines connected and drive away
  2. Disconnect the lines quickly to release the pressure
  3. Close the shut-off valves and bleed the pressure before disconnecting Correct answer.
  4. Cut the lines to relieve the pressure

Explanation

Pressurized air lines can whip and injure you when disconnected, so you close the shut-off valves and bleed the pressure first. Only then should you disconnect the lines by hand.

Question 48Coupling and Uncoupling

A driver couples a set and the test tug succeeds, but the rear trailer's lights do not work. What is the most likely cause?

  1. The electrical cord is not fully plugged in or is damaged Correct answer.
  2. The pintle hook latch is open
  3. The dolly landing gear is down
  4. The fifth wheel jaws are not locked

Explanation

Lights depend on the electrical cord being fully seated in the receptacle and undamaged. A loose or damaged cord breaks the circuit and leaves the rear trailer without lights, even when the mechanical coupling is sound.

Question 49Coupling and Uncoupling

When coupling a tractor to a converter dolly, what must be aligned and secured so the dolly's upper coupler plate couples correctly to the fifth wheel?

  1. The dolly's pintle hook must line up with the tractor's drawbar
  2. The dolly's kingpin must line up with the open fifth wheel jaws Correct answer.
  3. The dolly's electrical cord must be plugged in before the kingpin enters
  4. The dolly's spring brakes must be released before the plate contacts the fifth wheel

Explanation

A converter dolly attaches to the tractor through a kingpin on its upper coupler plate, exactly like a trailer, so the kingpin must be centered with the open fifth wheel jaws before backing under. The jaws then close and lock around the kingpin to hold the dolly. Air lines, cords, and brake status are handled after the mechanical coupling is secure.

Question 50Coupling and Uncoupling

You are coupling a second trailer to the rear of a lead trailer in a doubles set. After the kingpin is locked in the rear fifth wheel, which step comes next before you connect the air lines and electrical cord?

  1. Open the red trailer air supply valve to charge the rear trailer
  2. Raise the rear trailer's landing gear fully and secure the crank handle Correct answer.
  3. Release the tractor parking brake and drive forward to check tracking
  4. Perform the test tug with the trailer brakes released

Explanation

Once the rear trailer's kingpin is locked into the lead trailer's fifth wheel, the landing gear must be raised fully and its crank secured so it does not drag or strike the road. Only after the gear is stowed do you connect the air lines and electrical cord and then run the test tug. Driving forward or charging the system before the gear is up risks damage and an unsafe coupling.